Practical Applications

To date, exercise training literature does not typically differentiate between the performance level of athlete when discussing plyometric intensity. It has been recommended that elite athletes may tolerate greater volumes of work than novice athletes (Potach and Chu, 2000). Exercises such as those used in the present study are classified as high stress exercises which require 2-3 days recovery (Gambetta, 1998).

Previous studies have found relative PFs in plyometric exercises to be in the region of 3-4 times BW (Wallace et al., 2010, Ball and Scurr, 2009, Jensen and Ebben, 2007, Jarvis et al., 2016). These bilateral forces are comparable with the unilateral forces experienced during running (Nilsson and Thorstensson, 1989). As a result they most likely do not merit the classification of high stress exercises in non-elite populations. During the present study elite athletes produced forces in the order of 7x BW in bilateral plyometric challenges and 5x BW in unilateral plyometric tasks. These stark differences between populations demonstrate

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that the historical approach of categorising exercise intensities regardless of the level of athlete may be flawed.

The present results may be contrary to popular recommendations that novice athletes should perform lower volumes of work than elite counterparts (Potach and Chu, 2000). The relative intensity appears to be far higher in elite athletes in the present study in comparison with values reported within the literature (Jensen and Ebben, 2007, Wallace et al., 2010) and those observed in Chapter 3, therefore sessions which are matched in terms of contacts will differ greatly in terms of the mechanical stress accumulated and may prove more stressful for elite athletes. Under such circumstance it is reasonable to conclude that in fact the elite athlete should perform lower volumes than their recreational counterpart. Further research is required to explore the effects of contact matched sessions between elite and non-elite athletes on markers of fatigue and stress.

Resistance training exercises are typically described by the nature of the task, i.e. the load lifted multiplied by the number of repetitions. Despite the apparent logic of this system there may be significant variance between lifts of equal weight in terms time under tension and the subsequent implications on neuromuscular fatigue although total work performed appears to remain constant provided that range of motion does not vary (Tran and Docherty, 2006, McBride et al., 2009). When compared with results from the Chapter 3, the present findings demonstrate that athletes performing an identical plyometric task, such as completing a 40 cm drop jump, can produce significantly different outcomes when the populations are heterogeneous. Successful performance of jumping tasks demands both the skilful application of force as well as the underpinning strength qualities, both of which combine to determine joint stiffness to resist yielding on impact and to produce concentric force. Consequently, plyometric exercise intensity may be best classified by the outcome in the form of PF rather than the task itself. The trend of exercise intensity appears similar in this and the previous study, illustrating that the nature of the task does contribute to the potential for high forces to be applied. Therefore it appears legitimate to describe an exercise as having an “intensity opportunity”. This refers to the fact that certain exercises, such as progressively higher drop jumps, may present the athlete with an opportunity to use the impact to produce a greater PF. This is most likely to be observed during the impact and eccentric phases although successful augmentation of these phases may provide the

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opportunity for an augmented concentric phase through the increased utilisation of elastic energy and greater active state. However, taking advantage of this opportunity is dependent on the athlete’s ability to resist yielding and avoid dissipating increased forces which naturally follow increased falling velocity from greater drop heights.

Further to the requirement to differentiate exercise intensity between populations, high level programming may necessitate a differentiation between athletes within a population. Plyometric exercises involve a large skill component due to the rapid and complex multi- joint actions involved. Therefore the importance of motor specificity means that an athlete may demonstrate varied levels of competency across differing motor tasks (Nakata et al., 2010, Sale, 1988). The influence of motor skill means that training history and personal preferences are amongst a number of factors which dictate that the exercises in which an athlete shows the most skill will vary significantly (Eloranta, 2003). Furthermore, the nature of the exercise will also change the emphasis of the strength quality required to underpin performance. For example, a CMJ relies primarily on concentric strength during the concentric phase whereas reactive strength becomes more prominent in tasks such as drop jumping (Moritani, 1993, Earp et al., 2010, Earp et al., 2011, Beattie et al., 2017). This is further evidence that the historical approach within the literature to place an intensity rating on an exercise per se is highly flawed.

Naturally, with increased joint stiffness comes increased PF which the athlete must tolerate. The potential performance augmentation this increased intensity may offer must be reconciled with a greater systemic stress to the athlete and associated injury risk. This is in contrast to the objective of improving landing mechanics to reduce injury risk. In the latter scenario the reduction of impact force is desirable in order to decrease injury risk (Hewett et al., 2005a, Bates et al., 2013).

In Chapter 3 sEMG was used to classify exercises as ballistic or plyometric. Whilst elite athletes in the present study exhibited greater forces during the ballistic CMJ and SL-CMJ when compared to the non-elite athletes studied in Chapter 3, the magnitude of difference is much smaller than during plyometric comparisons. Therefore it would appear that it is the ability to generate force through elastic energy, underpinned by an ability to resist yielding

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through isometric and eccentric strength, which is the dominant source of greater intensity in elite populations as opposed to concentric force.

4.7

Limitations

The training background of the elite athletes within the present study are relatively homogenous, i.e. horizontal and vertical jumpers. This provides a reliable picture of this discrete population, however it is important to consider potential differences with other discrete groups who may also be described as elite athletes such as sprinters, middle distance runners and team sports athletes.

The assessment of reliability in this study is restricted to within session reliability rather than between session reliability. The rationale for this is the intention to demonstrate the capacity of variables to distinguish between exercises rather than of the athlete to reproduce consistent measures. Future studies applying this assessment system to monitor an athlete or group of athletes will require an understanding of between session reliability in order to evaluate meaningful differences.

4.8

Summary

The results of the present study provide unique and novel insight into the demands of plyometric and ballistic exercise within elite track and field athletes. Furthermore the findings describe a novel approach to analysing the demands of jump exercises by movement phase, thus providing a greater level of insight into the specific nature of stimulus. Exercise guidelines for this population may be informed by these findings and will allow coaches and athletes to prescribe training regimes with a greater degree of precision and clarity of likely stimulus.

It is also clear that a given plyometric exercise cannot be considered to have an inherent, fixed intensity. Instead it may only be regarded as having an “intensity opportunity” with the actual outcome being dependent on the fixed characteristics of the exercise and the variable neuromuscular qualities of the athlete performing it. Indeed, intensity opportunity can be considered both population specific and influenced by the skill and strength qualities

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of the athlete within a given population. The large forces observed during the landing phase of ballistic exercises highlights that coaches should give keen consideration to the total number and intensity of the landings accumulated within a session in contrast to the traditional view of simply counting impacts preceding a propulsion. Finally, whilst the absolute intensity of an exercise may vary between populations the present results demonstrate the capacity of plyometric exercises to elicit greater intensities than ballistic exercises during impact, eccentric and concentric phases and well as greater volume-loads for a single repetition as represented by impulse. Comparisons of bilateral and unilateral exercises are less clear owing to the absence of an accepted methodology for relative comparisons.

Having established and applied a methodology for evaluation of the magnitude of stimulus during jumping exercises, the focus of this thesis will now turn towards recent interest in the exploration of kinetic and kinematic characteristics which support elite jump performance.

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